This invention relates to a process for preparing film positive sheets and more particularly, to a process for preparing film positive sheets for prints bearing latent images which are normally invisible with the naked eye, but developed on copies of prints made from the film positive sheets whereby abuse of the prints such as forging or alternating thereof by copying machines can be prevented.
In order to prevent that important documents such as securities and secret documents are copies with intention of abuse, it has been proposed that warning marks such as "VOID" and the like that are invisible with the naked eye be previously printed on the documents yet distinguishable from the surrounding background when the documents are copied.
One proposal to date for attaining the above-mentioned purpose is a document in which the background is formed of fine meshes of a given size, and the warning mark is formed of meshes of a size different from that of the meshes forming the background. Each of the background and warning mark has a camouflage pattern incorporated therein so that the background and warning mark cannot be easily distinguished from each other with the naked eye.
Such prior art is disclosed in U.S. Pat. No. 4,265,459. Although the prior art provides the effect that the incorporation of the camouflage pattern in the warning mark makes it difficult notice of the presence of the warning mark with the naked eye, the prior art has not been widely applied because of the disadvantages which will be described hereinbelow.
First of all, the prior art is disadvantageous in that when the document is copied, the warning mark cannot be clearly noted on the obtained copy. That is, as the background and warning mark in the document are formed of circular meshes, it is required that one of the background and warning mark be formed of meshes having a size and the other of the background and warning mark be formed of meshes having a size different from the meshes forming the former. However, when the difference in mesh size is extremely great, even if the background and warning mark have camouflage patterns incorporated therein, there is the possibility that the warning mark may be identified with the naked eye. Thus, it has been contemplated that the meshes forming one of the background and warning mark be made larger than those forming the other of the background and warning mark in such a size relationship that the size of the larger meshes is a multiple of that of the smaller meshes whereby when the document is copied by a copying machine, the larger meshes are reproduced, but the smaller meshes are not reproduced in the obtained copy. However, with the multiple mesh size relationship between the larger and smaller size meshes, the size difference will not appear distinctly on the copy obtained from the document. Especially, because with rapid development of copying machine technology, the copy color adjusting range has been increased and presents the problem that the background and warning mark will be reproduced on document copies in substantially the same color tone.
And in the prior art described above, because the camouflage patterns as well as the background and warning mark are printed by a single screen, the camouflage patterns, warning mark and background lie in the same plane and do not exhibit any random appearance having decorative effects. Thus, prints or documents sometime have undesirable appearances and are not practical.
It is clear that the above-mentioned disadvantages are owing to processes for preparing film positive sheets from which prints or document are prepared.